Yeah it's not special effects, if anything it is the best voice for an animated movie. And the likely hood of Jim getting that? Who would have though Jim's voice winning an oscar? Not likely. If Jim were to win an oscar himself (not the movie team winning it - as in cg graphics, music) he would need to be in a non-animated role.

Otherwise the cast who did the actual making of the movie might win one, but Jim, Steve Carrol and the other voice actors will not get an oscar for this.

If they had a category of say best hero, and horton won it (I havent seen the movie but I'll assume for this example horton is a hero) then i'm not sure if the creators such as Dr Suess's wife would get it, or Jim. Considering that Jim was only the voice of Horton I assumed that Dr Seuss's wife would receieve the oscar for Dr Suess himself.